Geology Reference
In-Depth Information
During the past several decades, deserts have been advancing
across millions of acres of productive land, destroying range-
land, croplands, and even villages. Such expansion, estimated
at 70,000 km
2
per year, has exacted a terrible toll in human suf-
fering. Because of the relentless advance of deserts, hundreds
of thousands of people have died of starvation or been forced
to migrate as “environmental refugees” from their homelands
to camps, where the majority are severely malnourished. This
expansion of deserts into formerly productive lands is called
desertifi cation
and is a major problem in many countries.
Most regions undergoing desertifi cation lie along the mar-
gins of existing deserts where a delicately balanced ecosystem
serves as a buffer between the desert on one side and a more
humid environment on the other. These regions have limited
potential to adjust to increasing environmental pressures from
natural causes as well as human activity. Ordinarily, desert
regions expand and contract gradually in response to natural
processes such as climatic change, but much recent desertifi ca-
tion has been greatly accelerated by human activities.
In many areas, the natural vegetation has been cleared as
crop cultivation has expanded into increasingly drier fringes
to support growing populations. Because grasses are the
dominant natural vegetation in most fringe areas, raising
livestock is a common economic activity. However, increas-
ing numbers of livestock in many areas have greatly exceeded
the land's ability to support them. Consequently, the veg-
etation cover that protects the soil has diminished, causing
the soil to crumble and be stripped away by wind and water,
which results in increased desertifi cation.
One particularly hard-hit area of desertification is the
Sahel of Africa (a belt 300-1100 km wide, lying south of the
Sahara). Because drought is common in the Sahel, the region
can support only a limited population of livestock and
humans. Unfortunately, expanding humans (30 million peo-
ple in 1980, increasing to an estimated 50 million in 2000)
and animal populations and more intensive agriculture have
increased the demands on the lands. Plagued with periodic
droughts, this region has suffered tremendously as crops
have failed and livestock has overgrazed the natural vegeta-
tion, resulting in thousands of deaths, displaced people, and
the encroachment of the Sahara.
The tragedy of the Sahel and prolonged droughts in other
desert fringe areas remind us of the delicate equilibrium of
ecosystems in such regions. Once the fragile soil cover has
been removed by erosion, it takes centuries for new soil to
form (see Chapter 6).
There are many important reasons to study deserts and
the processes that are responsible for their formation. First,
deserts cover large regions of Earth's surface. More than 40%
of Australia is desert, and the Sahara occupies a vast part
of northern Africa. Although deserts are generally sparsely
populated, some desert regions are experiencing an influx
of people, such as Las Vegas, Nevada, the high desert area of
southern California, and various locations in Arizona. Many
of these places already have problems with population growth
and the strains it places on the environment, particularly the
need for greater amounts of groundwater (see Chapter 13).
Furthermore, with the current debate about global warm-
ing, it is important to understand how desert processes oper-
ate and how global climate changes affect the various Earth
systems and subsystems. By understanding how desertifi cation
operates, people can take steps to eliminate or reduce the
destruction done, particularly in terms of human suffering.
Learning about the underlying causes of climate change
by examining ancient desert regions may provide insight into
the possible duration and severity of future climatic changes.
This can have important ramifications in decisions about
whether burying nuclear waste in a desert, such as Yucca
Mountain, Nevada, is as safe as some claim and is in our best
interests as a society.
As an example, more than 6000 years ago, the Sahara
was a fertile savannah supporting a diverse fauna and fl ora,
including humans. Then the climate changed, and the area
became a desert. How did this happen? Will this region
change back again in the future? These are some of the ques-
tions geoscientists hope to answer by studying deserts.
And last, many agents and processes that have shaped
deserts do not appear to be limited to our planet. Features
found on Mars, especially as seen in images transmitted by
the
Spirit
and
Opportunity
rovers, are apparently the result of
the same wind-driven processes that operate on Earth.
Wind is a turbulent fl uid and therefore transports sediment
in much the same way as running water. Although wind
typically fl ows at a greater velocity than water, it has a lower
density and thus can carry only clay- and silt-size particles
as
suspended load.
Sand and larger particles are moved along
the ground as
bed load.
Sediments too large or heavy to be carried in suspension by
water or wind are moved as bed load either by
saltation
or by
rolling and sliding. As we discussed in Chapter 12, saltation
is the process by which a portion of the bed load moves by
intermittent bouncing along a streambed. Saltation also oc-
curs on land. Wind starts sand grains rolling and lifts and
carries some grains short distances before they fall back to
the surface. As the descending sand grains hit the surface,
they strike other grains, causing them to bounce along by
saltation (
◗
Figure 15.1). Wind-tunnel experiments show
that once sand grains begin moving, they continue to move,
even if the wind drops below the speed necessary to start
them moving! This happens because once saltation begins,
it sets off a chain reaction of collisions between sand grains
that keeps the grains in constant motion.
Saltating sand usually moves near the surface, and even
when winds are strong, grains are rarely lifted higher than
about a meter. If the winds are very strong, these wind-
whipped grains can cause extensive abrasion. A car's paint can
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